The cause for late-onset (sporadic) Alzheimer's disease (AD), an aging-related neurodegenerative disease, is unknown. Apolipoprotein E (apoE) is a major lipid transporter that exists in three isoforms (E2, E3, and E4) in human, coded by 3 distinct alleles ?2, ?3, and ?4. Both epidemiology and animal studies indicate that, beside age, APOE ?4 is a major genetic risk factor for AD, although the underlying mechanism remains largely elusive. Expression of apoE4 alone, even in old ages, however, is insufficient to cause AD, suggesting that other factors including environmental factors also play a role in the development of AD in this genetically predisposed population. Ozone (O3) is a highly reactive gas and one of the most abundant urban pollutants with over 30% of the population in the United States living in areas with unhealthy levels of O3. Although O3 is traditionally considered to be a lung toxicant, emerging evidence indicates that O3 inhalation also causes oxidative stress and pathological changes in other tissues/organs beyond the respiratory system. Interestingly, it has been reported that children and young adults living in urban areas with high levels of air pollution including O3 exhibited AD-like pathology in their brain. Our preliminary studies further show that cyclic O3 exposure, a situation that mimics human exposure, accelerates memory loss in APP/PS1 mice, a well-established animal model of AD, with no significant effect in wild type mice. We also show that O3 exposure induces oxidative stress and neuronal cell death in the cortex and hippocampus of APP/PS1 mice. Together, the data suggest that although O3 alone may not cause AD, it may increase the susceptibility of genetically predisposed population to AD. As oxidative stress contributes importantly to AD pathogenesis and increases with age, and as apoE4 mice show increased sensitivity to oxidative stress, we hypothesize that O3 exposure synergizes with genetic risk factor apoE4 and aging, leading to the development of late-onset AD. We will test our hypothesis in two specific aims using human apoE4 and apoE3 (represents the majority of human population who carry the APOE ?3 gene) targeted replacement (TR) mouse models. In Aim 1, we will test whether cyclic O3 exposure accelerates memory loss in apoE4 TR mice and whether aging will further exacerbate O3 effect. In Aim 2, we will test whether apoE4 TR mice are more sensitive to O3-induced oxidative stress, synaptic dysfunction, and neuronal cell death than apoE3 TR mice and whether aging further increases such sensitivity. The results from these studies will not only shed new lights on the etiology of AD but also build a foundation for further investigation of the interactions between O3 or other environmental risk factors, apoE4, and aging in the development of late- onset AD. The results will have major impact in the field and may lead to the development of novel strategies for the prevention and treatment of AD.